A secondary battery, which can be charged and discharged, has been increasingly used as a power source for medium- or large-sized devices, such as electric vehicles and hybrid electric vehicles, as well as small-sized mobile devices. When the secondary battery is used as the power source for the medium- or large-sized devices, a plurality of unit cells (secondary batteries) are connected in series or in series/parallel with each other so as to manufacture a battery module(s) providing high output. Consequently, the battery module is generally constructed in a structure in which the plurality of secondary batteries are electrically connected with each other.
Up to now, nickel-metal hydride secondary batteries have been widely used as the unit cells (battery cells) of the medium- or large-sized battery module. Recently, however, lithium secondary batteries have attracted considerable attention as the unit cells (battery cells) of the medium- or large-sized battery module because the lithium secondary batteries have high energy density and high discharge voltage.
The battery module is constructed in a structure in which a plurality of battery cells are included in the battery module. Consequently, it is necessary to detect the operation state of the respective battery cells and to control the operation of the respective battery cells. For example, it is necessary to detect physical operation states of the respective battery cells, such as the voltage and the temperature of the battery cells, and to secure the optimum operation state and the safety of the respective battery cells. The detection of the physical operation states of the respective battery cells is accomplished by a construction in which connecting members for voltage measurement, such as wires, are connected to the battery cells to be measured and a control unit and a construction in which temperature sensors are mounted at the outer surfaces of the battery cells to be measured or adjacent to the battery cells, and signals detected by the respective temperature sensors are transmitted to a control unit via connecting members.
As the number of the battery cells constituting the battery module is increased, therefore, the connection structure between the detection unit for detecting the physical operation state of the respective battery cells and the control unit is very complicated. In the construction for measuring the voltage of the battery cells, a process for detecting the voltage of the battery cells and transmitting the detected signals to the control unit is individually carried out. However, the measurement of the temperature of the respective battery cells must be improved.
When the lithium secondary batteries as described above are used as the unit cells of the battery module, on the other hand, the safety problem of the battery module must be more carefully considered. Each lithium secondary battery suffers relatively large volume change when lithium ions are absorbed to or discharged from anodes. Specifically, the lithium secondary battery is repeatedly expanded and contracted during the repetitive charge and discharge of the lithium secondary battery. The repetitive expansion and contraction of the lithium secondary battery increases the internal resistance of the lithium secondary battery. As a result, the efficiency of the lithium secondary battery is greatly reduced. Also, a battery case is separated from the remaining part of the lithium secondary battery due to the excessive expansion of the lithium secondary battery. As a result, an electrolyte leaks from the lithium secondary battery, and therefore, the lithium secondary battery may catch fire or explode. Furthermore, the fire or explosion of some of the battery cells results in the consecutive fire or explosion of the remaining battery cells, whereby a serious situation may be caused.
In a medium- or large-sized battery module using the lithium secondary batteries as the unit cells, therefore, it is necessary to detect the expansion state of the respective unit cells in consideration of the above-mentioned problems. Some conventional arts propose a technology for mounting a piezoelectric element or another unit to measure the expansibility of the battery cells to the surface of the battery cells.
When the above-mentioned detection unit and connecting members are further mounted to the battery module so as to detect the volume expansibility of the battery cells, however, the structure for detecting the physical operation state of the battery cells is further complicated.
For example, a construction for detecting a physical operation state of battery cells of a conventional medium- or large-sized battery module having a plurality of battery cells is illustrated in FIG. 1.
FIG. 1 schematically shows only the construction for obtaining a detected signal. As the number of physical operation states obtained from the battery cells is increased, however, the number of sensors is also increased. Furthermore, the number of detection circuits for electrically connecting the sensors is also increased with the result the structure of the battery module having a restricted size is further complicated.
In this connection, U.S. Pat. No. 6,137,262 discloses a technology for acquiring information on physical operation states from battery cells of a medium- or large-sized battery module having a plurality of battery cells connected in series with each other, converting the acquired information into coded alternating current voltage, and transmitting the coded alternating current voltage to a control unit via a power cable of the battery module. The technology disclosed in the U.S. Pat. No. 6,137,262 is embodied in a very simple structure because the signals detected from the respective battery cells are transmitted to the control unit not using an additional connecting member but using the power cable of the battery module. However, it is necessary to mount additional units for converting the detected signals into coded alternating current voltage to the respective battery cells. Consequently, the manufacturing costs of the battery module are inevitably increased. Furthermore, the structure of the battery module is further complicated due to the provision of the conversion units.
Also, Korean Patent Application Publication. No. 2001-46873 discloses a technology for measuring the voltage change of a plurality of thermocouples connected in series with each other and located at opposite ends of battery cells to detect the abnormal state of the battery cells while temperature sensors are not mounted to the respective battery cells. As clearly described in the specification of the Korean Patent Application Publication No. 2001-46873, the disclosed structure has an advantage in that the number of the temperature sensors, as detection members, is reduced, and therefore, a circuit for transmitting the detected signals is simplified. However, the disclosed structure has a fundamental problem in that the operation state information on the respective battery cells cannot be detected, and therefore, it is not possible to secure the substantial safety of the battery cells. In addition, the disclosure of the above-mentioned publication is limited to apply to a technology for measuring the expansibility of the respective battery cells of the battery module using the lithium secondary batteries as the unit cells.
Consequently, there is high necessary of a battery module having a new structure in which the construction for detecting information with respect to the respective battery cells and transmitting the detected signals to the control unit is simplified.